The present invention relates to a method for manufacturing a primary preform for an optical fibre, using a plasma chemical internal vapour deposition process, wherein doped or undoped glass-forming precursors are supplied to the interior of a hollow glass substrate tube, a reaction zone in the form of a plasma is moved back and forth along the length of the aforesaid hollow glass substrate tube between a point of reversal near the supply side and a point of reversal near the discharge side of the hollow substrate tube, wherein the substrate tube is positioned in a furnace and wherein such conditions are created in the aforesaid reaction zone that one or more glass layer packages made up of at least two separate glass layers are deposited on the interior of the aforesaid substrate tube. The present invention further relates to a method for manufacturing a final preform, to optical fibres as well as to primary preforms, final preforms and optical fibres obtained therewith.
In internal vapour deposition techniques, a reaction mixture consisting of glass-forming gases and optional dopants is supplied at the supply side of a hollow glass substrate tube, after which said gases are converted into glass in a reaction zone. Unreacted gases and/or residual products are discharged via the discharge side of the hollow glass substrate tube.
In an internal vapour deposition process of the PCVD (Plasma Chemical Vapour Deposition) type, the reaction zone is a plasma which is moved back and forth along the length of the hollow glass substrate tube. In a PCVD process, glass layers are directly deposited on the interior of the hollow glass substrate tube, independently of the direction in which the reaction zone is moving. A PCVD process is known, inter alia from U.S. Pat. Nos. 4,741,747, 5,145,509, 5,188,648, WO 2004/101458 and US 2008/0044150.
In an internal vapour deposition process of the MCVD (Modified Chemical Vapour Deposition) or FCVD (Furnace Chemical Vapour Deposition) type, the reaction of the glass-forming gases and optional dopants is activated by heating the exterior of the hollow glass substrate tube, using a burner or a furnace, respectively. In the reaction zone, which is located near the burner or the furnace, the glass-forming gases are converted into so-called soot, which soot is deposited on the interior of the hollow glass substrate tube under the influence of thermophoresis. Said soot is converted into glass by means of heating. In an MCVD or an FCVD process, glass layers are deposited only when the reaction zone is moving in the direction of the discharge side of the hollow glass substrate tube. PCVD, MCVD and FCVD process are known in the art.
JP 57-51139 discloses an MCVD process in which a starting material for an optical fibre is produced. In a cycle, a number of glass layers are deposited on the interior of a substrate tube, with the deposition starting at a position near the supply side and the distance along which the reaction zone moves in the direction of the discharge side varying with each glass layer. The starting material is produced by carrying out a number of cycles in succession.
An optical fibre consists of a core and an outer layer surrounding said core, also referred to as “cladding”. The core usually has a higher refractive index than the cladding, so that light can be transported through the optical fibre.
The core of an optical fibre may consist of one or more concentric layers, each having a specific thickness and a specific refractive index or a specific refractive index gradient in radial direction.
An optical fibre having a core consisting of one or more concentric layers having a constant refractive index in radial direction is also referred to as a (multiple) step-index optical fibre. The difference ni between the refractive index of a concentric layer and the refractive index ncl of the cladding can be expressed in a so-called delta value, indicated Δi% and can be calculated according to the formula below:
            Δ      i        ⁢                  ⁢    %    =                              n          i          2                -                  n          cl          2                            2        ⁢                  n          i          2                      *    100    ⁢    %                  where:        ni=refractive index value of layer i        ncl=refractive index value of the cladding        
An optical fibre can also be manufactured in such a manner that a core having a so-called gradient index refractive index profile is obtained. Such a radial refractive index profile is defined both with a delta value ΔA % and with a so-called alpha value α. The maximum refractive index in the core is used for determining the Δ% value. The alpha value can be determined by means of the formula below:
      n    ⁡          (      r      )        =                    n        1            ⁡              (                  1          -                      2            ⁢            Δ            ⁢                                                  ⁢            %            ⁢                                          (                                  r                  a                                )                            α                                      )                    1      2                      where:        n1=refractive index value in the centre of het fibre        a=radius of the gradient index core [μm]        α=alpha value        r=radial position in the fibre [μm]        
A radial refractive index profile of an optical fibre is to be regarded as a representation of the refractive index as a function of the radial position in an optical fibre. Likewise it is possible to graphically represent the refractive index difference with the cladding as a function of the radial position in the optical fibre, which can also be regarded as a radial refractive index profile.
The form of the radial refractive index profile, and in particular the thicknesses of the concentric layers and the refractive index or the refractive index gradient in the radial direction of the core determine the optical properties of the optical fibre.
A primary preform comprises one or more preform layers which form the basis for the one or more concentric layers of the core and/or part of the cladding of the optical fibre that can be obtained from a final preform.
A preform layer is built up of a number of glass layers. In an internal vapour deposition process, a glass layer is the layer that is deposited upon movement of the reaction zone from the supply side to the discharge side or from the discharge side to the supply side.
A final preform as referred to herein is a preform from which an optical fibre is made, using a fibre drawing process.
To obtain a final preform, a primary preform is externally provided with an additional layer of glass, which additional layer of glass comprises the cladding or part of the cladding. Said additional layer of glass can be directly applied to the primary preform. It is also possible to place the primary preform in an already formed glass tube, also referred to as “jacket tube”. Said jacket may be contracted onto the primary preform. Finally, a primary preform may comprise both the core and the cladding of an optical fibre, so that there is no need to apply an additional layer of glass. A primary preform is in that case identical to a final preform. A radial refractive index profile can be measured on a primary preform and/or on a final preform.
The length and the diameter of a final preform determine the maximum length of optical fibre that can be obtained from the final preform.
To decrease the production costs of optical fibres and/or increase the yield per primary preform, the aim is therefore to produce a maximum length of optical fibre that meets the required quality standards, and that on the basis of a final preform.
The diameter of a final preform can be increased by applying a thicker layer of additional glass to a primary preform. Since the optical properties of an optical fibre are determined by the radial refractive index profile, the layer of additional glass must at all times be in the correct proportion to the layer thickness of the preform layers of the primary preform that will form the core, more in particular the one or more concentric layers of the core, in the optical fibre. Consequently, the layer thickness of the glass layer additionally applied to the primary preform is limited by the thickness of the preform layers being formed by means of the internal vapour deposition process.
The length of a final preform can be increased by increasing the length, more in particular the usable length, of a primary preform. The term “usable length” is to be understood to be the length of the primary preform along which the optical properties remain within predetermined tolerance limits, which tolerance limits have been selected so that optical fibres that meet the desired quality standards are obtained.
To determine the usable length of the primary preform, a radial refractive index profile is measured at a number of positions along the length thereof, after which it is possible, based on said measurements, to determine a so-called longitudinal refractive index profile and a longitudinal geometry profile for each preform layer, if desired.
Thus, a longitudinal refractive index profile can be considered to be a graphic representation of the refractive index of a preform layer as a function of the longitudinal position in the primary preform. It is also possible, of course, to use the refractive index difference rather than the refractive index for determining a longitudinal refractive index profile.
A longitudinal geometry profile can be considered to be a graphic representation of the thickness of the cross-sectional area of a preform layer as a function of the longitudinal position in the primary preform. The cross-sectional area, also referred to as CSA, can be calculated on the basis of a radial refractive index profile. The CSA can be calculated as follows:
      CSA    i    =            π      4        ⁢          (                        d                      i            ,            u                    2                -                  d                      i            ,            i                    2                    )                      where        CSAi=cross-sectional area of the preform layer i [mm2]        di,u=external diameter of the preform layer i [mm]        di,i=internal diameter of the preform layer i [mm]        
The usable length of a primary preform is in particular adversely affected by so-called “taper”. The term “taper” is to be understood to be a deviation of the optical and/or geometric properties of the primary preform in regions near the ends thereof. A distinction is made between optical taper and geometric taper.
Optical taper relates to deviations of the refractive index (or the refractive index difference), whilst geometric taper relates to deviations of the cross-sectional area of the preform layer.
If a primary preform is built up of several preform layers, the optical and geometric taper of the preform layers differ from each other.
Methods for reducing optical and/or geometric taper are known in the art.
U.S. Pat. No. 4,741,747, for example, discloses a method for manufacturing optical preforms according to the PCVD method, wherein glass layers are deposited by causing a plasma to move back and forth between two points of reversal in the interior of a glass tube, with the addition to the tube of a reactive gas mixture at a temperature ranging between 1100° C. and 1300° C. and a pressure ranging between 1 hPa and 30 hPa. By causing the plasma to move non-linearly as a function of time near at least one of the points of reversal, the magnitude of the region exhibiting non-constant deposition geometry at the ends of the optical preform is reduced.
The present inventors have found that such a method leads to a reduction of the geometric taper, to be true, but that the optical taper does not improve, or even worsens. Moreover, the present inventors have found that it is in some cases necessary to influence the refractive index of the deposited glass also at other positions outside the so-called taper regions.
Although it is thus possible, using the prior art methods, to increase the usable length of a primary preform, there is a need for a method by means of which the usable length can be increased even further.